The workprinter triggers frame capture that takes place in a stop-motion software called "cinecap". Cinecap is made to capture DV frames and assemble them on the fly into .avi. You can't just hook a different sort of camera to it because the images won't be compatible with the software. Even if they were, if there were more resolution it wouldn't be able to keep up (already it requires a Raid-0 disk array so it doesn't lose frames). The more recent HD models have additional special hardware that enables it to handle the greater data throughput.
There would be an improvement in video quality in HD. But not enough for me to justify the expense right now.
Remember that the quality can always be improved. If I were to switch to HD, the next question would be, why not do an RGB transfer? Then, why not do a native scope transfer? Then, why not a wet transfer? How about a 2K - no, 4K transfer? For mortals like us, there is always a step up. We could probably spend $50K transferring these grungy 16mm films if we really want to. My whole philosophy for doing them is so that they are available as research reference material for the forum members, and I'm delighted to see them getting frequent use in this capacity. I never had imagined that people would want to watch them (although that's cool that some people do).
I happened to have already had this telecine setup... I didn't buy it for this purpose (although I have invested in several modifications for it because of the SW projects). If someone wants to invest in the equipment necessary to make the very best possible transfer of the 16mm prints, that's fine, but I can't justify it given that in the end even the very best 16mm print is still going to be a grossly cropped, grainy, low-resolution image, in some cases with lint burned right into the emulsion.